Lymphopenia, a lower-than-normal level of lymphocytes in the blood, has a wide range of causes including infections, medications, autoimmune diseases, nutritional deficiencies, and bone marrow problems. In adults, it’s defined as fewer than 1,500 lymphocytes per microliter of blood, with normal counts falling between 1,000 and 4,800. For children age 6 and younger, the threshold is higher: below 2,000 per microliter.
Some causes are temporary and resolve on their own, while others signal a chronic condition that needs attention. Here’s a breakdown of the most common reasons lymphocyte counts drop.
Infections That Deplete Lymphocytes
Viral infections are among the most frequent triggers. HIV is the most well-known example, as it directly destroys a specific type of lymphocyte (CD4 T cells), progressively weakening the immune system over time. But many other infections also cause lymphopenia, including influenza, viral hepatitis, COVID-19, tuberculosis, malaria, and sepsis (a severe, body-wide response to infection).
With most acute infections like the flu or COVID-19, the drop in lymphocytes is temporary. The body pulls lymphocytes out of the bloodstream and sends them to infected tissues to fight off the pathogen. This redistribution makes the count on a blood test look low even though the cells are still active in the body. Counts typically recover as the infection clears. In a study of hospitalized COVID-19 patients, malnourished individuals were over 7 times more likely to have persistently low lymphocyte counts, suggesting that the body’s ability to bounce back depends partly on nutritional reserves.
Medications and Medical Treatments
Several categories of drugs lower lymphocyte counts as a direct side effect. Chemotherapy is the most obvious, as it kills rapidly dividing cells throughout the body, including the immune cells produced in bone marrow. Radiation therapy has a similar effect, particularly when directed at areas near large amounts of bone marrow like the pelvis or chest.
Long-term use of corticosteroids (often prescribed for asthma, arthritis, or inflammatory conditions) actively destroys lymphocytes and pushes them out of the bloodstream. This is one reason people on high-dose steroids are more vulnerable to infections. Other immunosuppressant drugs, including those used after organ transplants and certain antibody-based therapies, deliberately suppress lymphocyte activity or numbers to prevent the immune system from attacking transplanted tissue or the body’s own cells.
Autoimmune and Inflammatory Diseases
In autoimmune conditions, the immune system mistakenly attacks healthy tissue, and sometimes lymphocytes themselves become targets. Systemic lupus erythematosus (lupus) is one of the strongest examples. Lymphopenia is so common in lupus that it’s actually one of the criteria used to help diagnose the disease.
Rheumatoid arthritis, myasthenia gravis, and sarcoidosis can also drive lymphocyte counts down. The mechanisms vary: in some cases the disease process destroys lymphocytes directly, in others the medications used for treatment are responsible, and sometimes both factors contribute simultaneously. Hodgkin lymphoma, a cancer of the lymphatic system, can cause lymphopenia through a combination of the disease replacing healthy immune tissue and the chemotherapy used to treat it.
Stress Hormones and Cortisol
Physical stress, whether from surgery, severe burns, intense exercise, or critical illness, triggers a surge of stress hormones that reshuffles where immune cells go in the body. Cortisol, the body’s primary stress hormone, is especially potent at moving lymphocytes out of the bloodstream and into tissues like the skin, lungs, and lymph nodes.
Research on how stress hormones affect immune cell distribution shows that cortisol significantly decreases circulating lymphocyte numbers, including both T cells (which coordinate immune responses) and B cells (which produce antibodies). The hormone works by changing molecules on the surface of lymphocytes that act like address labels, redirecting cells away from the blood and toward tissues where they might be needed to fight infection or heal wounds. This is why a blood test taken during or shortly after a stressful event can show a temporarily low lymphocyte count.
Cushing syndrome, a condition in which the body produces too much cortisol over a long period, can cause chronic lymphopenia through the same mechanism.
Nutritional Deficiencies
Your body needs adequate protein and certain micronutrients to produce lymphocytes. Protein-energy malnutrition, the kind seen in severe dieting, eating disorders, chronic illness, or food insecurity, directly reduces the bone marrow’s ability to generate new lymphocytes. Zinc deficiency is particularly relevant because the thymus gland, where T cells mature, depends heavily on zinc to function.
This connection between nutrition and lymphocyte production has practical consequences. In one study of early-stage COVID-19 patients, malnutrition was identified as an independent risk factor for low lymphocyte counts, with malnourished patients having roughly three times the odds of lymphopenia compared to well-nourished patients. The researchers noted that malnutrition is an important but generally unrecognized cause of lymphopenia.
Bone Marrow Problems
Since lymphocytes are produced in the bone marrow, any condition that damages or crowds out normal marrow tissue can reduce lymphocyte output. Aplastic anemia, where the marrow stops producing enough blood cells of all types, is a classic example. Leukemia and lymphoma can infiltrate the marrow and displace the stem cells responsible for making lymphocytes.
Inherited bone marrow failure syndromes, though rare, are another cause. Conditions like Fanconi anemia and dyskeratosis congenita involve genetic defects that impair the marrow’s ability to produce blood cells normally. These typically present in childhood and carry an elevated risk of developing leukemia or other cancers later in life.
Protein Loss and Gastrointestinal Disease
Conditions that cause the body to lose large amounts of protein through the gut, known collectively as protein-losing enteropathy, can also lead to lymphopenia. Inflammatory bowel disease, severe celiac disease, and certain heart conditions like constrictive pericarditis can trigger this type of protein loss. When proteins leak out of the intestinal wall faster than the body can replace them, the raw materials needed to build new lymphocytes become scarce. Chronic kidney disease has a similar draining effect on immune cell populations.
Why the Level Matters
Not all lymphopenia carries the same risk. A mildly low count (just under 1,500 in an adult) during an acute illness is usually temporary and resolves without intervention. Persistently low counts or severely low counts signal something more significant.
A large Danish study tracking over 98,000 people found that the risk of serious infection climbed steadily as lymphocyte counts dropped. The highest risk group, male smokers over age 80 with counts below 500 per microliter, had a 42% chance of being hospitalized for an infection within two years. Even in less extreme cases, lymphopenia served as a meaningful predictor of future infection risk, suggesting that a persistently low count deserves investigation rather than a wait-and-see approach.
In children, the thresholds for concern are higher than in adults. Infants under two weeks old normally have lymphocyte counts above 2,000 per microliter, and counts that would be considered normal in a 40-year-old could actually represent lymphopenia in a toddler. Age-specific reference ranges matter when interpreting a child’s blood work.